I can think of two ideas on top of my head:

- Similar sympthoms but different disease (flu?)

- False negatives, and so people were still actually sick

In addition, I wonder if those patients had distinct clinical features compared to others. In absence of more data (and these cases are very uncommon) it is hard to tell.

Probably a good idea would be to do a study with periodical follow up of a large cohort of discharged patients. This would probably give better indication.